To pull an Amy

When someone is the most hyped about a new plan (or something in the course of being established) but then, also the first to flake.

Unreliable, characterized by not following through on what he/she says (gets easily excited about things, and as a result over promises, only to underdeliver...)

“Gosh! X just pulled an Amy again! She (X) told me one of her colleagues went climbing, and that now she is definitely going to do it. But when I invited her for a session, she was ‘busy’”

"It's so easy for you to pull an Amy!"